The present invention relates to an optical reimaging system. More specifically, the invention relates to an infrared optical reimaging system which incorporates a plurality of lens sets to transfer to one or more detector arrays an image produced by an imaging system, such as a telescope. This reimaging system can incorporate a number of small detector arrays to be used instead of a single large array of detectors. The lenses in the reimaging system can be arranged to provide reduction of the image if necessary to accommodate the small detector arrays.
The principal, though not exclusive, application of the present invention is in infrared detection and image transmissions. Among the principal concerns of infrared optical systems is "noise" produced in the final image by warm radiation emanating from the imaging system itself. A second concern is preventing stray rays from outside the desired field of view from becoming part of the final image produced by the system.
It is therefore desirable in infrared imaging systems to reduce noise generated by warm radiation emanating from the system itself and to reduce the amount of stray warm radiation appearing in the image to be viewed. It is well known that warm radiation from the imaging system itself can be prevented or reduced by "cold shielding". Cold shielding refers to the physical cooling of all or part of an optical system by various known refrigeration means. Effective cold shielding can be had by cooling only that part of the system between the detectors and nearest aperture stop or baffle, since the aperture will itself protect the final image from radiation emanating from system parts between the aperture and the field of view. In addition, the aperture itself should be cooled to reduce the amount of warm radiation emanating from it.
Use of an aperture stop in the system provides one means for eliminating or reducing the stray warm radiation from outside an infra-red imaging system, as shown in the U.S. Pat. No. 3,963,926 to Borrello and U.S. Pat. No. 4,583,814 to Koetser. The Borrello patent describes a system wherein an image produced by a primary optical system strikes separate detectors in a large detector array. Interposed between the detector array and the primary system is an overlay with a plurality of apertures--one aperture corresponding to each detector. In accordance with the Borrello system, each detector in the detector array "sees" only one segment of the image due to the configuration of the array and walls which separate the individual detectors.
Significant problems appear to exist with Borrello and other prior art systems which use large detector arrays to detect an image produced by a primary optical system, i.e., a scanner or telescope. Detector arrays are made, in at least one form, of rows and columns of discrete detectors. It is known in the industry that the proportion of defects in the detector array increases in proportion to the square of detectors in the array. For such high density arrays, manufacturing yields are low because of the increased chance of having at least one defective detector in the array. Large detector arrays also have problems with placement of the leads (conductors connecting the detectors to the sensing circuitry). Power requirements can also become restrictive. Optical baffling between detectors is not easily accomplished. Further, individual detector arrays remain susceptible to blinding and damage from laser threats principally in military applications. The present system reduces or eliminates these problems by permitting the use of smaller detector arrays, for instance ones having 16.times.16 detectors in an array as compared to the 128.times.128 detectors in arrays used in some current systems.
A further problem with the Borrello and other systems is that as the size of the focal plane increases, the focal length must also increase to maintain the same optical characteristics. As a consequence, efficient cold shielding requires much greater cooling of larger portions of the optical trains of such systems. The present system can accommodate increases in focal plane size by addition of lens sets identical to those already in the system, and no increase in focal length is necessary.